Hydrocarbons may be produced from wellbores drilled from the surface through a variety of producing and non-producing formations. The wellbore may be drilled substantially vertically or may be an offset well that is not vertical and has some amount of horizontal displacement from the surface entry point. In some cases, a multilateral well may be drilled comprising a plurality of wellbores drilled off of a main wellbore, each of which may be referred to as a lateral wellbore. Portions of lateral wellbores may be substantially horizontal to the surface. In some provinces, wellbores may be very deep, for example extending more than 10,000 feet from the surface.
A variety of servicing operations may be performed on a wellbore after it has been initially drilled. A lateral junction may be set in the wellbore at the intersection of two lateral wellbores and/or at the intersection of a lateral wellbore with the main wellbore. A casing string may be set and cemented in the wellbore. A liner may be hung in the casing string. The casing string may be perforated by firing a perforation gun or perforation tool. A packer may be set and a formation proximate to the wellbore may be hydraulically fractured. A plug may be set in the wellbore.
Perforation tools may comprise explosive charges that are detonated to fire the perforation tool, perforate a casing if present, and create perforations and/or tunnels into a subterranean formation proximate to the wellbore. The tunnels into the subterranean formation may be surrounded by an envelope or layer of crushed material. The crushed material may shift and/or flow into the tunnels, clogging the tunnels to some extent, reducing the ease with which hydrocarbons can flow from the subterranean formation, through the perforations in the casing, and up the wellbore. Additionally, metal material in the perforation gun, such as a portion of a tool body of the perforation gun proximate to the explosive charges and/or a charge liner may be deposited in the tunnels, potentially partially clogging the tunnels. On net, it is desired that perforation not only opens the casing to allow hydrocarbons to flow into the wellbore but further increases the flow rate of hydrocarbons relative to what would have been the flow rate if the casing was merely opened without creating tunnels in the subterranean formation. As indicated above, however, the flow rate of hydrocarbons may be increased or decreased relative to the flow rate that would theoretically be experienced by a tunnel-less subterranean formation as a result of the perforation. A variety of perforation tool design parameters can be adjusted with the intention of encouraging desired results and mitigating undesired results of the perforation. A density of shots into the subterranean formation can be adjusted. An angle of the focus axis of the explosive charges can be adjusted to angle up, to angle down, or to angle normal to the axis of the perforation tool, though an angle normal to the axis of the perforation tool may provide the greatest degree of penetration of the formation. Parameters of the explosive charge itself may be altered to adapt to different downhole parameters including the shape of the charge, the amount of explosive material, the construction of a explosive charge liner, a design of the perforation gun tool body proximate to the explosive charges, and other parameters.
Sometimes downhole perforation procedures are conducted with pre-firing wellbore pressure maintained below the formation fluid pressure, which may be referred to as an under balanced condition, or maintained above the formation fluid pressure, which may be referred to as an over balanced condition. Sometimes a tool may be carried with the perforation tool or incorporated into the perforation tool to create a transient fluid surge after firing of the perforation tool to supplement or prolong an under balanced condition. The perforation procedure may be designed to adapt to different perforation conditions based on estimations and/or projections of downhole parameters. For example, a pre-firing wellbore pressure may be calculated to provide a specific intensity of under balance or over balance. In the case an under balance condition is desired, a volume of fluid surge may be calculated.
In order to design the perforation tool and/or a downhole perforation procedure, one or more rock cores that are considered to be representative of the subterranean formation to be perforated may be tested to determine some parameters of the subterranean formation and/or interactions between the explosive charges and the subterranean formation. The evaluations of test results may be used in designing the perforation tool and/or the downhole perforation procedure.